In recent years, following the diffusion of computers, inkjet printers are widely utilized not only in offices but also for the purpose of printing on papers, films, cloths, etc. in households.
The inkjet recording method includes a system of discharging droplets by applying a pressure by piezoelectric elements; a system of discharging droplets by generating bubbles in an ink by heat; a system of using ultrasonic waves; and a system of suction discharging droplets by an electrostatic force. As ink compositions for such inkjet recording, aqueous inks, oil based inks, or solid (melt type) inks are used. Of these inks, aqueous inks are the mainstream in view of production, handling properties, odor, safety, and other properties.
Coloring agents that are used for these inkjet recording inks are required such that they have high solubility in solvents; they can achieve high-density recording; they have a good hue; they have excellent fastness to light, heat, air, water, or chemicals; they have good fixing properties to image receiving materials and hardly cause bleeding; they have excellent fastness as inks; they have no toxicity; they have a high purity; and additionally, they are available cheaply. However, it is extremely difficult to find out coloring agents capable of meeting these requirements at high levels.
Various dyes and pigments for inkjet use have already been proposed and actually used. However, it is a present state that any coloring agent capable of meeting all of these requirements has not been discovered yet. Conventionally well known dyes or pigments to which color index (C.I.) numbers are given are difficult to cope with both hue and fastness as required in inkjet recording inks.
So far, food black based dyes, naphthol black direct azo dyes, acid azo dyes, and so on have been widely known as inkjet black dyes.
As food black based dyes, C.I. Food Black 1 and C.I. Food Black 2 are representative, and techniques regarding use of these inkjet black inks are described in JP-A-2-36276, JP-A-2-233782, JP-A-2-233783, etc.
Also, techniques for use of C.I. Acid Black 2, C.I. Acid Black 31, C.I. Acid Black 52, C.I. Acid Black 140, C.I. Acid Black 187, etc. as acid azo dyes for inkjet black inks are described in JP-A-60-108481, JP-A-2-36277, JP-A-2-36278, etc.; and techniques for use of C.I. Direct Black 9, C.I. Direct Black 17, C.I. Direct Black 38, C.I. Direct Black 51, C.I. Direct Black 60, C.I. Direct Black 102, C.I. Direct Black 107, C.I. Direct Black 122, C.I. Direct Black 142, C.I. Direct Black 154, C.I. Direct Black 168, etc. as direct azo dyes for inkjet black inks are described in JP-A-56-139568, JP-A-61-285275, JP-A-3-106974, etc.
In general, it is difficult to cover the hue of black by single use, and therefore, a combination with a short-wave dye is preferably carried out.
As the short-wave dye, direct azo dyes, acid azo dyes, etc. are similarly widely known. Techniques for use of C.I. Acid Yellow 17, C.I. Acid Yellow 23, C.I. Acid Yellow 49, C.I. Acid Yellow 194, etc., or C.I. Direct Yellow 861, C.I. Direct Yellow 120, C.I. Direct Yellow 132, C.I. Direct Yellow 144, etc. for inkjet black inks are described in JP-A-7-97541, WO97/16496, JP-A-10-158560, JP-A-11-12514, etc.
The dye of the invention is more excellent in fastness than these dyes, and techniques for use of the dye of the invention as an inkjet black ink are described in JP-A-2003-306623.
The present inventors made investigations regarding inkjet inks using a dye. However, it was noted that there is a problem that aqueous black inks are low in image durability. In particular, in the case of expressing a gray continuous pattern with monochromatic images, it was considered problematic that deterioration of gray images in low-density areas is remarkable.
Also, different from carbon black, etc., black dyes capable of giving black images having a high printing quality such as letters by single use are not substantially existent, and it is required to mix and use several kinds of dyes having a different absorption spectrum. However, such dyes are liable to be decomposed depending upon the ink preparation condition and dye kind. It has been noted that inks using such dyes lack ink stability, resulting in causing a problem that discharge failure of ink is liable to occur.